In a magnetron described in Patent Document 1, in order to interpose a circularly annular yoke plate in which small projections for small air gap formation are scattered, folding processing is applied so as to form an air gap between a magnetic pole piece (hereinafter called a pole piece) and the sintered magnet (hereinafter called an annular magnet). Patent Document 1 discloses a technique for interposing the circularly annular yoke plate in the air gap between the pole piece and the annular magnet in order to prevent electromagnetic waves generated at the time of operating the magnetron from leaking to the outside of an apparatus.